We study the transcriptional control of T cell development and function. T cells are essential for immune responses. Most recognize peptide antigens presented by class I (MHC-I) or class II (MHC-II) classical Major Histocompatibility Complex molecules, and express either of two surface glycoproteins that contribute to antigen recognition: CD4, which binds MHC-II, or CD8, which binds MHC-I. Consistent with such binding properties, MHC I-specific T cells generally express CD8 but not CD4 (CD8 T cells), whereas the opposite is true of MHC II-specific T cells (CD4 T cells). CD4 and CD8 T cells differentiate in the thymus from 'double-positive' precursors that express both CD4 and CD8. Because of their pivotal role in immune responses, the differentiation of CD4 T cells has remained a key area of focus in the laboratory. We had previously shown that the transcription factor Thpok promotes the differentiation CD4 T cells in the thymus and their 'pre-programming' for helper functions, and supports the stability of mature CD4 T cells, the latter in part redundantly with the related transcription factor LRF (Leukemia-Lymphoma Related Factor). We had also shown that Thpok and LRF are essential for the differentiation and function of 'regulatory' CD4 T cells expressing the transcription factor Foxp3 (Treg cells), that are essential for immune tolerance. Ongoing analyses focus on additional functions of Thpok in CD4 T cell differentiation and responses, and on the molecular mechanisms that underpin this broad array of Thpok functions. In addition to their distinct antigenic specificity, CD4 and CD8 T cells perform different functions upon antigen encounter. CD4 T cells differentiate into multiple effector subtypes, typically defined by their production of specific cytokines, including Th1 (making IFN-gamma [IFN-g]), Th2 (IL-4), and Th17 (IL-17). In contrast, the differentiation of antigen-stimulated CD8 T cells is strongly skewed towards IFN-g production and acquisition of cytotoxic functions; such 'Tc1' cells can kill other cells, including cells infected by intra-cellular pathogens, but also cancer cells. The latter property is exploited, directly or not, by most current cancer immunotherapy strategies. Of note however, CD8 cells retain the ability to differentiate into IL-17 producers (Tc17 cells, that make little or no IFN-g). Differentiation into such effector subtypes is induced by signals accompanying antigen encounter, most notably cytokines produced and surface ligands expressed by antigen-presenting cells. These signals promote the expression of subtype-specific transcription factors, including T-bet, Eomes and Runx3 for Tc1 and Th1 cells, and Stat3 and a T cell-specific isoform of ROR-gamma (RORgt) for Tc17 and Th17 cells. Our studies of Thpok functions have recently led us to unexpected observations on the differentiation of IL-17 producing cells, which are involved in responses to extra-cellular pathogens, including bacteria and fungi. We and others had previously shown that Thpok serves in CD4 T cells at least in part by inhibiting the expression of the transcription factor Runx3, and thereby of genes involved in T cell cytotoxicity. Contrasting with these findings, we observed that Thpok does not repress Runx3 in Th17 cells, and is not needed for their differentiation altogether. This suggested that the 'transcriptional circuitry' of Th17 cells inhibits expression of cytotoxic genes independently of Thpok. We verified that this was also the case in Tc17 CD8 T cells, which express little or no Thpok. In additional in vivo and in vitro analyses, we found that repression of cytotoxic genes is an intrinsic property of the Th17/Tc17 circuitry, which acts by inhibiting the function but not the expression of Runx3. Inhibition of cytotoxic gene expression requires Stat3, in part through its ability to promote RORgt expression. However, while RORgt itself represses genes encoding IFN-g or cytotoxic molecules (e.g. granzymes or perforin), it fails to do so for genes encoding Th1 transcription factors T-bet or Eomes. Rather, efficient repression of both genes requires active Stat3. Stat3 activation is initiated and maintained by inflammatory cytokines (including IL-6, which promotes Th17/Tc17 differentiation). Thus, these findings imply that persistent inflammatory signals are needed for efficient repression of the Th1/Tc1 transcriptional circuitry in Th17/Tc17 cells; that is, upon cessation of inflammation, Th17/Tc17 cells would revert to a Th1/Tc1 status. We proposed that this contributes to the previously observed instability of IL-17-producing T cells. Conversely, these findings have potential implications in the tumor micro-environment, in which persistent inflammation would result in Stat3 activation and repression of cytotoxic functions, and thereby could reduce the antitumor potential of CD8 T cells. Indeed, others had found that Stat3 disruption promotes responses against experimental tumors and suggested that such an effect was mediated by increased cytotoxic gene expression.